| A short treatise about TZE batteries and charging [message #356071] |
Thu, 25 June 2020 11:32 |
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Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
Karma: 7
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Senior Member |
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While GMC's motorhome was not nearly the first in the marketplace, it was first in many areas and this is obvious when you compare the developments across the production years.
In the early years, batteries were cheap and copper was expensive, that changed in 1975 when copper went to historic lows.
The early years had both the house and the main engine batteries in the right front. There was a single battery in the rear to crank the APU. There is only one advantage to this if you do not know the costs. With this arrangement, if one was careless or ignorant enough to kill both the main engine and house bank, he could still crank up the APU and recover.
There was also a relatively severe problem with the #10 fore to aft conductor. First off, it isn't even a real #10. It is #10SAE and that makes it about 90% if a #10AWG. This means its ampacity is also 90% and its forward voltage drop is 110%. When charging lead/acid batteries that have their entire working life between 11.9 and 12.7 volts (those can vary a little) that voltage drop can make or break a system.
In the early coaches, it was a bust because the period standard ferro-resonant converter was built for a fixed output voltage that could not not overcome the line loss built into the coach electrics. I knew this was a problem, but it was only after I installed a real battery monitor that I identified that that wire run was a lot of the issue. We have dry camped at more than a few summer music festivals. With the combination of not being able to create more than a moderate recharge and the electric refrigeration, the APU would often run 40 hours on a weekend. The shore power charging loss is less of an issue with the house bank in the rear. Even when upgraded to a pair of GC2s. That loss is still not a big issue.
Even with a new PD9140+wizard, the best charge current I could make was about 27 amps and that would trail off well before the converter was ramping down. This was the result of a 0.8V drop in the system between the converter and the bank. I did some careful measurements at that time. Those proved educational. One tends to think of the copper conductors as all that matters (unless he has been working in fiberglass boats). Well, I was getting a 0.3V loss on the negative side and no amount of cleaning connections could improve that. If you are not willing to make some serious mods, you are stuck with that.
There are two positive sides to the house bank in the front. First is that the boost function works real well (small victory) and the second is that the main engine alternator can throw more power at it because of the low cable loss. This last one is yet another two edged thing. After I kept burning belts when leaving the a fore mentioned weekend dry camps, I tried to improve the belt tension to absolute maximum. With this, we still burned belts if I tried to move the coach too soon (like an half an hour) after start-up. I knew from my marine work that 100amps for a nominal 15volt system (~2hp) was the absolute limit for a single V belt but I did not believe that the 80amp version of 27SI alternator mounted could do that. Well, I was wrong. It seems that the combination of shorter heavier cable and the pair of GC2s made more of a difference than I had expected. I put a DC current probe on the alternator lead after simulating a weekend dry camp at home. With the engine ramped to 1800 (secondary road speed), the alternator output would hold at 96~98 amps for as long as I dared (I could smell hot belt). At startup, the meter had trapped a peak reading of over 100 amps. The 27SI didn't seem to mind. So that was when I elected to go the dual belt route.
These issues will not arise in the later 26' builds. With the current limit imposed by the #10 cable, I suspect that the 80amp alternator is never overloaded. But, unless you have a very late coach with the upgraded fore to aft cable you cannot do a lot for a dead main engine battery is a real hurry. Even without the larger cable, you can bring a lot of power to the front battery if you are patient. That is a small problem as the later coaches also have a momentary switch for this function. I can understand why GM did that. If you really need to get the main engine started and that battery is low. Fire up the APU. If you have a good multistage converter it will most likely jump right in at boost mode (~14.2), then the line loss to the main engine battery matters less. If everything else is in decent shape, it should take lass than 10 minutes until you can crank your main engine.
If you have read all the way to here, you may well have a question that I did not answer. If you do, contact me and if I can't answer it right away, I can search up the now 10+ year old notes from the experiments. If there is no answer there, maybe I need to do another experiment or two. I don't mind, because we all learn something.
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
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